1. Field of the Invention
This invention relates to a method and device for stabilizing a deteriorating spinal motion segment composed of two vertebrae adjacent a disc.
2. Description of the Related Art
Low back pain is a medical condition affecting millions of humans. It is the most common cause of disability for the middle aged working population. In addition to the discomfort and pain experienced by the individual, substantial costs are borne by society, including costs for the diagnosis and treatment, and the cost of payments for disability benefits. Lost productivity is more difficult to quantify, but we know that it is substantial. Any new knowledge that leads to a greater understanding of the causes, prevention, or treatment of the low back pain is a worthwhile and important contribution to the welfare of individuals and society in general. Any intervention that reduces or eliminates symptoms would constitute a significant breakthrough. That discovery would improve the quality of life for countless individuals and reduce the expenditure of scarce societal financial resources.
Recent medical evidence indicates that the symptoms of low back pain are most commonly caused by pathologic changes occurring in what is called the "spinal motion segment." The spinal motion segment consists of a unit of spinal anatomy bounded by two vertebral bodies, containing those bodies and the interposed intervertebral disc, as well as the attached ligaments, muscles, and the facet joints.
The disc consists of the cartilaginous end plates at the top and bottom of the vertebral bones, the annulus fibrosis running circumferentially around the nucleus, and the nucleus itself. In normal discs, the nucleus acts as a damper to cushion applied loads, thus protecting the other elements of the motion segment. The nucleus contains hydrophilic (water attracting) mucopolysaccharides and a small amount of fibrous strands. The nucleus is incompressible. It responds to compression forces (as in sitting, standing walking and lifting) by bulging outward against the vertebral end plates and the annulus fibrosis. Mechanical forces acting on the disc are substantial, i.e. approaching 1000 lbs/in.sup.2 when a heavy load is lifted.
The annulus consists of a woven fabric of collagen fibers and a smaller amount of elastic fibers, both of which are extremely strong in resisting tension forces. Unfortunately the annulus is not very strong when it is required to withstand compression and shear forces. The vertebral end plate can deform slightly by bulging into the vertebral bodies, as long as the applied forces are not great, and a long as the force is dampened by the bulging of the annulus.